Injection molding has long been used for manufacturing articles from thermoplastic and thermosetting polymeric resins. Small articles having relatively complex or detailed geometry are particularly well suited for manufacture by injection molding. Such articles can include, for example, polymeric bottle caps, cutlery, blow molding preforms, cottage cheese tubs; and other articles that are similarly suited to manufacture by a redundant injection molding operation. In a conventional injection molding process, a polymeric molding powder is introduced into the heating chamber of the machine, where it is plasticized. The plasticized polymer is then injected into the mold cavity. The rheological properties of the polymer must be such that it will flow readily through the sprue and mold gate and fill the mold uniformly. The injected polymer is maintained inside the mold under high pressure until it is sufficiently cooled, whereupon the mold is opened and the finished article is ejected.
The productivity that can be achieved in a repetitive molding operation is directly related to the time required to complete one cycle of operation. The overall time required for one operating cycle will necessarily depend upon a variety of factors including, for example, the composition and physical properties of the resin, molding temperature, mold geometry, and the like. Generally speaking, however, the time required to cool the injected polymer prior to ejecting the molded article from the mold comprises a major portion of the overall cycle. The mold is typically cooled by circulating fluid through a network of internal channels. When polymer is injected into the mold, that portion of the polymer which contacts the mold is cooled first, thereby forming a skin on the surface of the molded article. A temperature gradient is established which extends outwardly from the relatively hotter core of the article toward the relatively cooler skin at or near the walls of the mold cavity. Cooling is continued within the cavity until such time as sufficient heat has been dissipated from the molded articles that they will not deform or stick together upon being ejected from the mold.
Particular problems have been encountered in injection molding olefinic resins. Such resins tend to shrink away from the mold surface during cooling. Because the mold cavities are necessarily vented, air fills the space between the mold walls and the molded articles and forms an insulating barrier that hinders cooling and further lengthens the required cooling time.
The extent of cooling that is required prior to ejecting articles from the mold therefore depends not only upon the molding machine design, composition and physical properties of the polymeric resin, the weight and geometry of the molded articles and the like, but also upon the apparatus and methods by which they are handled upon being ejected from the mold. When the molded articles are initially ejected from the injection mold, the outer skin is desirably thick enough to resist deformation as they fall from the mold into a collecting bin, onto a conveyor, or the like. Once the molded articles are no longer in thermal contact with the cooled mold surface, the thermal energy at the core continues to migrate toward the relatively cooler skin, and if heat is not dissipated from this skin at a rate that is at least as great as the rate of migration from the core, the skin will begin to soften, making the article more susceptible to deformation and sticking. This effect is particularly significant when molding polymeric articles having walls with relatively thick cross sections.
With conventional injection molding equipment, the molded articles must be retained in the mold until sufficient thermal energy has been dissipated through the surfaces adjacent to the chilled mold walls that the surfaces will not be softened to an undesirable extent by the additional thermal energy which migrates to the surface after ejection. Even where the skin is not heated sufficiently to cause major deformation, it may become tacky or sticky, causing adjacent parts to stick or clump together during handling and storage. The foregoing problems have previously been avoided by extending the cooling portion of the injection molding cycle, thereby increasing cycle time and decreasing productivity.
In recent years, robots have been used by some for removing molded articles from the mold cavities of injection molding machines. However, such robots are expensive to build and maintain, and their use still does not permit the cycle time to be reduced to the extent desired.
An apparatus and method are therefore needed that will permit injection molded polymeric articles to be ejected from the mold cavity sooner than is now possible, and that will permit the molded articles to be transported and further cooled without risk of deformation or sticking.